The field emission device (FED) was initially studied and developed for use as an electron emission source suitable mainly for the flat panel display (FPD) type image display device to replace the classical thermionic emission type cathode ray tube (CRT). In recent times, a need has started to be felt for a field emission device with the capability to adequately focus the electron beam emitted from the emitter tip so as to be suitable also as an electron beam lithography electron source or a FPD requiring ultrahigh definition.
As a field emission device studied in response to this, there is known a field emission device with built-in focusing electrode, generally known by the abbreviated name “double-gate type,” which, as taught by Document 1 indicated below, is not only provided with an extraction gate electrode around the emitter tip but is additionally equipped with a focusing electrode (lens electrode) for focusing the electron beam. In the case of this type of field emission device with built-in focusing electrode, referred to as an “FEA with built-in lens,” the extraction gate electrode and the focusing electrode are both configured to have openings (desirably circular openings as perfectly round as possible) that expose the tip of the emitter formed on the substrate to the space above. Therefore, in the sense that these electrodes surround the emitter, they are from the shape aspect called ring electrodes.    Document 1: “Fabrication of Silicon Field emitter arrays Integrated with beam focusing lens”, Yoshikazu Yamaoka et al., Jpn. J. Appl. Phys., Vol. 35, Part 1, No. 12B, (1996) pp. 6626-6628.
With regard to the focusing electrode, this Document 1 sets out three configurations, (a)-(c), in its positional relationship with the extraction gate electrode.
(a) Structure in which the focusing electrode is provided above the extraction gate electrode.
(b) Structure in which it is provided in the same plane to surround the extraction gate electrode.
(c) Structure in which it is stacked on top of the extraction gate electrode but the rim of the extraction gate electrode opening rises in the vertical direction like a conide (Fujiyama-shaped/conical) volcano crater to enter the opening of the focusing electrode in an upwardly mounded shape, whereby the height of the rim of the focusing electrode opening becomes substantially the same as the height of the rim of the extraction gate electrode opening.
In the case of a field emission device with built-in focusing electrode which has at least a focusing electrode in addition to an extraction gate electrode, when the emitter potential is made 0 V, for example, a certain positive voltage Vex is of course applied to the extraction gate electrode in order to extract electrons. A voltage Vf at least lower than Vex (Vf<Vex) is applied to the focusing electrode in order to focus the emitted electron beam. Although the focusing effect is naturally stronger as Vf is lower, the amount of current that can be extracted from the emitter decreases markedly if Vf is lowered to near 0V. This is because the electric field concentration at the emitter tip is relaxed by the voltage Vf lower than Vex, with the result that the field strength applied to the emitter tip weakens.
In order to overcome this problem, a scheme has been devised whereby, as seen in Document 2 indicated below, the position of the rim of the focusing electrode opening is set lower than the position of the rim of the extraction gate electrode opening so as to keep the low potential distribution produced by the focusing electrode from reaching the emitter tip, thereby obtaining an emitted electron beam focusing effect while maintaining the field strength applied to the emitter tip.    Document 2: “Focusing Characteristics of Double-Gated Field-Emitter Arrays with a Lower Height of the Focusing Electrode”, Yoichiro Neo et al., Appl. Phys. Exp. 1 (2008), 053001-3.
However, even with such a structure, when it is attempted to achieve a stronger focusing effect, the potential barrier of low potential produced by the focusing electrode is still formed above the emitter tip, so that part of the emitted electron beam undesirably returns to the gate without being able to go beyond the potential barrier, thus posing another problem of the extractable amount of current again decreasing.
Therefore, it was attempted to avoid a potential barrier from being formed on a line perpendicular to the emitter tip which is the electron emission point by providing still another focusing electrode stage and applying a positive voltage thereto. In FIG. 2 of Document 3 indicated below and FIG. 9 of Document 4 indicated below, structures having two focusing electrodes are shown.    Document 3: Unexamined Japanese Patent Publication H7-192682    Document 4: Unexamined Japanese Patent Publication H6-275189
However, electric field calculation and electron trajectory computer simulation carried out earlier by the present inventors found that whilst a device structure having two focusing electrodes as focusing lenses does in fact enable formation of a focused electron beam, the field concentration at the emitter tip is lost and the amount of discharged current decreases. In other words, a potential distribution to the focusing electrodes that enables electron beam focusing without loss of the electric field on the emitter tip could not be found within the range of voltages that can be applied to an actual device.
So the present inventors also considered a field emission device with built-in focusing electrode structured to include another focusing electrode so as to have a laminated structure with a total of three focusing electrodes. The reason was that they thought that by this, even when applying a potential low enough to satisfy the focusing effect at the intermediate second focusing electrode, it might be possible for the lowermost first focusing electrode to prevent the so-caused relaxation of the electric field concentration of the emitter tip and be further possible for the uppermost third focusing electrode to prevent a potential barrier from being formed on a line perpendicular to the electron emission point.
From verification results, such a structure was in fact determined to obtain satisfactory characteristics as the device electrical characteristics. However, a problem was next encountered from the aspect of fabrication method. Specifically, it was found that when such a three-fold focusing electrode structure is adopted, an efficient electron beam focusing effect cannot be obtained unless the intermediate second focusing electrode is given a considerably large film thickness of, say, 1 μm or greater as compared with the approximately 200 nm that suffices for the other electrodes. But when it is attempted to form on the same substrate such a structure wherein only the second focusing electrode is thick, such a structure cannot be favorably fabricated no matter which of the various fabrication methods so far reported is applied.
In order to resolve this problem, some of the inventors proposed in Document 5 indicated below, which was filed as Japanese Patent Application 2008-218897, a rational device production method and a field emission device, such as shown in FIG. 4, of a structure obtained by stacking at least four stages of focusing electrodes of substantially the same order of thickness. Including the extraction gate electrode of the lowermost stage, the stacked electrode structure came to have five stages in total.    Document 5: Unexamined Japanese Patent Publication 2010-55907
FIG. 4(B) is a plan view of an example of such a field emission device, and (A) of the same figure is a sectional end view along line 4A-4A of the figure. An emitter 11 constituting a sharply pointed electron emission terminal is formed on a substrate 10 by a tip 11tp, and in order to expose at least the tip 11tp of this emitter 11, an insulating film 12 is provided on the substrate 10, and on this is formed an extraction gate 13 which by application of a suitable voltage (bias voltage) promotes electron emission from the emitter tip 11tp. 
A stacked focusing electrode structure 20 constituting a focusing lens with respect to the emitted electron trajectory is built above the extraction gate electrode 13. When the unit stacked stage is defined as one insulating film layer and one focusing electrode layer formed thereon, the stacked focusing electrode structure 20 is configured by stacking at least four or more of these unit stacked stages in the direction perpendicular to the substrate 10, and in the illustrated case consists of four stages. Where the lowermost stage, i.e., the focusing electrode 21 located at the lowest position in the vertical direction, is called the first focusing electrode, a second focusing electrode 22, third focusing electrode 23 and fourth focusing electrode 24 are stacked upward in order via first˜fourth insulating films 25˜28, respectively.
As shown in FIG. 4(B), the extraction gate electrode 13 and the first to fourth focusing electrodes 21˜24 all have openings as seen from above in plan view, and these openings are generally most desirably circular openings. Therefore, as seen in the sectional end view of FIG. 4(A), the insulating films 12, 25˜28, and the electrodes 13, 21˜24, are all provided so as to surround the emitter 11 while being spaced apart from the emitter 11 in the radial direction.
In other words, as regards the insulating films 12, 25˜28, the inner peripheral edges 12e, 25e˜28e of their openings, and as regards the electrodes 13, 21˜24, the inner peripheral edges 13e, 21e˜24e of their openings are the respective portions of closest to the emitter 11 as viewed in the radial direction. Further, the sectional configuration resembles the shape near the crater of a conide (Fujiyama-shaped/conical) volcano, and the vicinity of the openings 12e, 25e to 28e: 13e, 21e˜24e are all shaped to be upwardly mounded above the plain below.
In comparison with not only the conventional device of two or fewer focusing electrodes but also with the device having three focusing electrodes that is impractical from the aspect of fabrication method, the field emission device with built-in focusing electrode in which the four focusing electrodes 21˜24 are stacked in this manner can satisfy the required condition of a fundamental structure enabling thoroughly practical fabrication, while greatly improving freedom of how potential is imparted, giving rise to freedom and accuracy in electric field distribution control, and basically overcoming the risk of electron current decline, electron reversal, and the like.
In such a structure, Document 5 teaches that for obtaining optimum electric field concentration, the vertical positions of the tip 11tp of the emitter 11 and the inner peripheral edge 13e of the extraction gate electrode 13 are desirably given the same height or the emitter tip 11tp is made about 0.1 μm higher, and/or, as shown by dimensions d1˜d4, the inner peripheral edges 25e˜28e of the insulating films 25˜28 are desirably set back somewhat more in the radially outward direction than the inner peripheral edges 21e˜24e of the electrodes 21˜24 respectively on top of themselves.
As collision of the emitted electrons with the insulating films 25˜28 degrades the dielectric strength voltage of these portions, giving rise to a risk of leakage current occurrence and lowering reliability, the latter is for preventing this, and since collision of emitted electrons with residual gas molecules before arriving at an anode electrode not shown in the drawings ionizes the gas molecules, so that generated positive ions are accelerated toward the emitter 11 in the opposite direction from the electrons to eventually collide with some part of the structure built on the substrate 10, is for ensuring that such collision does not arise because should the collision occur at the insulating film, it will again lead to degradation of the dielectric strength voltage. As is well known, when the voltage applied to the anode electrode is on the order of several kV, it is far higher than the voltages applied to the extraction gate electrode 13 and the focusing electrodes 21˜24, so that the positive ion trajectory becomes substantially perpendicular to the substrate 10 irrespective of the voltage applied to the extraction gate electrode 13 and the focusing electrodes 21˜24. Therefore, in order to prevent the positive ions from colliding with the insulating films 25˜28, it is necessary to set the insulating films 25˜28 to positions where the insulating film inner peripheral edges 25e˜28e are not visible when looking at the device from vertically above. Therefore, in the case of a configuration wherein, as illustrated, the electrode opening diameter decreases with lower electrode position, it is, in line with this, better to define the setback distance larger (make the setback distance longer) as the insulating film is lower and nearer to the emitter 11, i.e., is better to define d1>d2>d3>d4.
Further, since it is troublesome if, for example, electric field concentration at the focusing electrode 22 and third focusing electrode 23 becomes so great as to cause field emission therefrom, to avoid this, electron emission is impeded by increasing the work function of at least the electrodes where field emission is probable, or as indicated taking the third focusing electrode 23 as representative and enlarging the peripheral edge 23e at the portion of FIG. 4(A) enclosed by a phantom line, it is considered preferable to avoid a sharp angle at the joining edges between the electrode surfaces and the face of the peripheral edge 23e orthogonal thereto by processing the surface of the opening peripheral edge to a smooth shape having no angle, e.g., to a sectionally semicircular shape.
As clarified later herein, the present invention teaches another improved configuration from a viewpoint different from Document 5 explained above, but it is noted beforehand that when the present invention is applied to a device of sectional structure similar to the field emission device shown FIG. 4, the various considerations set out in the foregoing can be applied without modification also in the field emission device to which the present invention is applied.
At any rate, it goes without doubt that the provision of the field emission device shown in FIG. 4 overcomes or at least mitigates the various drawbacks and disadvantages of earlier field emission devices. The electron beam emitted from the emitter can be thoroughly focused without reducing the extractable amount of electron current, freedom of how potential is imparted to the electrodes is greatly improved, and freedom and accuracy of electric field distribution control is realized. In other words, it can be said that there was provided a fundamental structure for applying desirable bias voltage for ensuring electron current and enabling electron beam focusing.
However, even the field emission device shown in FIG. 4, which is far superior to earlier ones, was found as a result of studies carried out by the present inventors to still have a problem that needs to be resolved. This can be explained with reference to the simulation results of FIG. 5. In this figure, symbols the same as those in FIG. 4 indicate the same constituent elements, but as indicated by the portion Ee enclosed by a phantom line circle, the trajectory of those among the electrodes emitted from the tip 11tp of the emitter 11 that pass near the outer peripheral edge of the focusing lens constituted by the focusing electrodes 21˜24 is markedly curved compared with the trajectory of the electrons passing through the lens center region and thus becomes an electron trajectory Edsp that is a source of aberration giving rise to spherical aberration.
As aberration of the electron beam is of course undesirable, it needs to be prevented, and it is conceivable to interpose an opening structural member, classically called an aperture, to block or bounce back the electron trajectory Edsp that is the cause of aberration. Even in a field emission device of a sectional structure such as shown in FIG. 4(A), it is not impossible to configure an aperture by, for example, minimally designing the opening diameter of the focusing electrode 21 immediately above the extraction gate 13 or the other focusing electrodes 22˜24. However, when electrodes collide with the electrode constituting the aperture, the impact causes gas to discharge from the electrode. When the discharged gas causes electrical discharge to occur between the electrodes, particularly with the emitter, it leads to immediate device destruction. This is especially true in the case of an intricate field emission device fabricated using fine processing technology down to the nano-order. Since this is something that must absolutely be avoided, the upshot becomes that it is not practical to use one of the stacked electrodes also as an aperture.
In this regard, what can be equally said not only about the field emission device shown in FIG. 4 but also about the field emission devices known heretofore is that little observation and consideration have been made with respect to the form of equipotential lines (two-dimensionally equipotential planes) in the vicinity of the tip 11tp of the emitter 11, i.e., with respect to potential distribution.
Specifically, in this type of field emission device, equipotential lines are formed in shapes following the outer surface contour of the emitter 11, as shown in FIG. 6, and the electrons emitted from the emitter tip 11tp are accelerated perpendicular to these equipotential lines. This situation does not change no matter how the potential of the extraction gate electrode 13 is varied. It can be seen that in this case, when electrons are emitted right on the center axis, they are accelerated straight along the center axis to make the desirable electron trajectory Ec, but when they deviate even slightly from the center axis, they are accelerated in a direction departing from the center axis. Thus, the electrons accelerated in an outwardly inclined direction from the center axis come to be emitted along the electron trajectory Edsp causing spherical aberration. Note that while FIG. 6 is a simulation diagram where the emitter potential was set at 0 V and the potential of the extraction gate electrode 13 at 50 V, even under other potential conditions the nonparallel equipotential lines ordinarily remain as generated in the vicinity of the emitter tip 11tp, and these become the primary cause of spherical aberration.